CHICAGO (Reuters) - Most people know eating broccoli is good for you but it also can help skin cells fend off damage from harmful ultraviolet radiation, U.S. researchers said on Monday.

The extract derived from newly sprouted broccoli seeds reduced skin redness and damage by more than one-third compared with untreated skin, they said. The extract already has been shown to help skin cells fight UV damage in mice.

"This is a first demonstration that a human tissue can be protected directly against a known human carcinogen," said Dr. Paul Talalay of Johns Hopkins University, whose study appears in the Proceedings of the National Academy of Sciences.

"This is not a sunscreen," Talalay said in a telephone interview. Instead, the extract helped fortify skin cells to fight the effects of UV radiation.

Unlike sunscreens, which provide a physical barrier against UV rays by absorbing, blocking or scattering the light, the extract helped boost the production of protective enzymes that defend against UV-related damage, Talalay said.

He has been studying sulforaphane -- a compound in broccoli sprout extract -- for more than 15 years. It has been shown to prevent tumor development in a number of animals treated with cancer-causing agents.

Talalay and colleagues tried it on six people, testing different doses of the extract on several small patches of skin, which was then exposed to a short pulse of UV radiation sufficient to cause varying degrees of sunburn.

They compared the redness of the skin in the treated and untreated areas. "That redness is a measure of a series of processes that go on in the skin which are harmful, including DNA damage," Talalay said.

At the highest doses, the extract reduced redness and swelling by an average of 37 percent.

The effect was long-lasting, Talalay said. "Two days after we stopped treatment, there was still an effect," he said.

The effect varied widely among the volunteers, ranging from 8 percent to 78 percent protection, due to genetic differences.

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"What we have shown is important because it works in humans," Talalay said. "How it should be applied to humans -- that requires further work."

The extract might be useful as a means of protecting against exposure to UV radiation, especially in people with suppressed immune systems who are most at risk for skin cancer, such as transplant patients, Talalay said. But it is no substitute for sunscreen.

"It does not prevent the radiation from penetrating into skin cells," he said.

Skin cancer is the most common cancer in the United States, affecting more than 1 million Americans every year, according to the National Cancer Institute. It kills more than 10,000 people each year, representing about 4 percent of all cancer deaths.